The United States today faces major challenges to its national security — long-term with Russia and China and immediately with regard to North Korea and Iran. Iran, in particular, can serve as a model for turning these grave challenges into constructive approaches and opportunities.Why is Iran a threat to the interests of the United States and its allies in the Middle East and what is the United States doing about it? The danger of Iran getting a nuclear weapon is a serious concern for the United States and the world. Yet President Donald Trump has indicated that he may kill the very international accord that would prevent Iran from getting one. Such American action would re-open the door to an Iranian nuclear bomb and put the United States in violation of an international pact. With a stroke of his pen, Trump could undermine the credibility of the United States as a partner to its closest allies and increase the stature and influence of its largest competitors (Russia and China), and place in doubt American leadership in the world. Moreover, such a dramatic step back from the deal would send a message to North Korea that the United States cannot be relied upon to meet its commitments — a message we can ill afford to send.The White House holds that Iran is in violation of the deal. This finding is in contradiction of assurances from international monitors that Iran is in full compliance with its commitments to a greatly reduced, limited and comprehensively monitored nuclear program.Ending U.S. support for the nuclear deal would signal a new, more assertive military posture toward Iran. The administration argues that the agreement did not place firm limits on Iran’s non-nuclear behavior, particularly its testing of ballistic missiles; support for proxy military forces in the region; and the violation of the human rights of its citizens. But the six world powers that negotiated the deal agreed at the outset that the core objective was to prevent Iran from getting a nuclear weapon.While concerns remain about Iran’s behavior, the negotiators understood that any pursuit of an "all or nothing deal" would have slowed, if not killed, progress on preventing Iran from acquiring a nuclear weapon. North Korea demonstrates that an armed nuclear capability gives a small nation outsized power to threaten and destabilize far beyond its borders.The inscrutable challenge remains how best to push back against Iran’s non-nuclear threats without enabling them to rush for a nuclear weapon. So far, the Trump policies appear to be: eliminate the nuclear agreement; mount a more focused military partnership with Israel and Saudi Arabia against Iran’s proxy military activities; increase American armed forces in the region and refocus on Iran instead of ISIS as America’s main enemy. I believe this approach would lead to another war with all the high costs and unacceptable consequences we have come to experience in this war-torn part of the world. There is a better way to achieve America’s objectives while still containing Iran.• First, the Trump administration should boldly take ownership of the nuclear agreement with a pledge to build upon it to prevent an Iranian nuclear weapon over the long term. The administration should negotiate with its European allies on how the nuclear deal can be made more effective, especially regarding its "sunset" provisions that reduce restrictions on Iran’s nuclear program by 2025. • Second, the United States should collaborate with Europe and other nations in the Middle East to seek a regional approach to missile testing. Despite lacking a nuclear warhead, as Iran improves its the accuracy of its missile arsenal, it becomes more threatening. • Third, the United States should draw on this complex and groundbreaking agreement as a model for a worldwide approach to uranium enrichment and plutonium separation in support of nuclear non-proliferation. • Finally, the most significant change I would make would be to develop an integrated political, military and diplomatic approach that would form a more united front of allies to push back against Iran while creating new opportunities for peaceful solutions to the lengthy wars that have ravaged parts of the Middle East.We must include Iran in that diplomatic initiative, since without Iran no regional solution can be achieved. In strained and dangerous moments, it is essential to talk to your enemy — as the adage goes, "keep your friends close and your enemies closer."Thomas Pickering is a former U.S. under secretary of state for political affairs and ambassador to Israel, Russia and the United Nations. He will deliver the keynote address at the St. Petersburg Conference on World Affairs tonight on "The Challenges and Opportunities in American Foreign Policy Today." For details about the conference, which continues through Friday, go to stpetersburgconferenceonworld affairs.com.